Viscosity controlled wellbore fluids comprising gels, such as viscoelastic surfactant gels, are used for various purposes in wellbore operations, such as drilling, completion, production and sequestration or other operations, particularly during hydrocarbon recovery operations. These viscosity controlled fluids must be adapted to form high viscosity fluids for operations such as fracturing, but must also be adapted for “breaking” or reducing their viscosity for subsequent operations, such as hydrocarbon recovery operations.
Fracturing fluids are complex and must simultaneously provide high temperature stability at high pump rates and fluid shear rates that may tend to cause the fluids to degrade and prematurely settle out constituents, such as proppant, before the fracturing operation is complete. Various fracturing fluids have been developed, but most commercially used fracturing fluids are aqueous-based liquids or fluids that have either been gelled or foamed using a gelling agent. Polymeric gelling agents, such as solvatable polysaccharides that are gelled by crosslinking to increase viscosity have been used. Non-polymeric viscoelastic surfactant (VES) gelling agents have also been used. In many cases, VES materials are advantageous compared to polymer gelling agents because they employ low molecular weight surfactants rather than high molecular weight polymers and may leave less gel residue within the pores of oil producing formations, leave no filter cake on the formation face and minimal amounts of residual surfactant coating the proppant, and inherently do not create microgels or fish-eye-type polymeric masses.
VES materials also require breaker systems for the non-polymeric VES-based gelled fluids to reduce their viscosity after use. These have generally included using external or reservoir conditions for viscosity reduction (breaking) and VES fluid removal (clean-up) during hydrocarbon production, as well as rearranging, disturbing, and/or disbanding of the VES worm-like micelle structure by contact with hydrocarbons within the reservoir, more specifically contacting and mixing with crude oil and condensate hydrocarbons. While useful, these breaker systems have limitation, including incomplete removal of the VES fluids, resulting in residual formation damage (e.g., impairment of hydrocarbon production). Post-treatment clean-up fluids composed of either aromatic hydrocarbons, alcohols, surfactants, mutual solvents, and/or other VES breaking additives have been employed in an attempt to break the VES fluid for removal, but their effectiveness has been limited, resulting in well sections with unbroken or poorly broken VES-gelled fluid that impairs hydrocarbon production, or in production delays associated with instances where VES breaking and clean-up takes a long time, such as several days up to possibly months to break and then produce the VES treatment fluid from the reservoir.
Internal breakers that are activated within the fluid, such as by downhole temperatures have also been used with VES-gelled fluids, and typically allow a controlled rate of gel viscosity reduction in 1 to 8 hours, similar to gel break times common for conventional crosslinked polymeric fluid systems. VES-gelled fluids are not comprised of polysaccharide polymers that are easily degraded by use of enzymes or oxidizers, but are comprised of surfactants that associate and form viscous rod-shaped or worm-shaped micelle structures. Conventional enzymes and oxidizers have not been found to act and degrade the surfactant molecules or the viscous micelle structures they form. Other internal breakers for VES-gelled fluids have been proposed in U.S. Pat. No. 7,595,284 B2 to Crews, which describes aqueous fluids viscosified with viscoelastic surfactants (VES) that may have their viscosities affected (i.e., reduced or broken) by the indirect or direct action of a composition that contains at least one metal ion source and optionally at least one second source. An optional second source may be a chelating agent where at least one reducing agent source may be additionally optionally used. Another optional component with the metal ion source includes a second different metal ion source. The breaking composition is believed to directly attack the VES itself, possibly by disaggregating or otherwise attacking the micellar structure of the VES-gelled fluid through chemical alteration of the VES compound and/or possibly by changing the chemical structure of the VES to give two or more products.
While these internal breakers are very useful, the development of additional internal breakers to provide enhanced control of the breaking of VES fluids is very desirable, particularly in view of the widespread use of these fluids in fracturing and other downhole operations.